A liquid crystal layer of a twisted nematic liquid crystal with an electric field applied in the vertical direction to the substrates has been used as a display device. In this type of display, two polarizer plates, the polarizing axes of which are crossed to each other, are arranged on the both dies of the liquid crystal layer. When the electric field is applied, the liquid crystal molecules are orientated in the vertical direction, in order to obtain a black image. However, with liquid crystal molecules aligned in the vertical direction, light transmitted through the liquid crystal layer at an oblique angle is double refracted by the liquid crystal molecules, rotating polarizing direction, so that when an operator views the LCD device at the oblique angle, the display of full black image is not obtained. This narrows the viewing angle in which the operator can see the displayed image with the desired high contrast.
To solve the described problem, it has been proposed that the LCD device be operated with the IPS mode in which the direction of the electric field applied to the liquid crystal is in parallel to the substrates. It is known that, in the IPS mode, since the liquid crystal molecules are mainly rotated in a plane parallel to the substrates, the difference in the degree of the double refraction, or birefringence, when the electric field is applied and when it is cut off is reduced providing a wider viewing angle.
One of methods for improving the viewing angle and tint of an LCD device operated in the IPS mode, uses an optical compensation material having the birefringent characteristic disposed between the liquid crystal layer and the polarizer plate. In a Japanese published application 5-505247 based upon a PCT filing, an IPS mode optoelectronic liquid crystal switching element is disclosed in which a birefringent optical compensation component is disposed between the substrates and the polarizer. In a Japanese Published Unexamined Patent Application 9-80424, a birefringent medium is disposed between the substrates and the polarizer plate LCD device operating in the IPS mode. The angle between a polarizing axis of the polarizer plate and a retardation axis direction of the birefringent medium is 20.degree..about.60.degree., preferably 30.degree..about.50.degree.. This solves the problem of having a white color image or a gray scale image is cast with a yellow or a blue tint, when the image is viewed from the oblique angle.
The IPS mode of operation has one principle visual characteristic defect. The IPS mode uses the liquid crystal molecules having a homogeneous structure arranged in the horizontal direction and two polarizing plates, their transmission axis cross each other at 90.degree. directed in an upper/lower direction and a right/left direction with respect to the front of the display surface. When the display surface is viewed at an oblique angle in the upper/lower direction and the right/left direction, the two transmission axis can be seen to cross each other at right angles, and the birefringence in the liquid molecules of the homogeneous structure is less than that of a twisted mode liquid crystal layer. As a result, a sufficient contrast is realized. However, when the display surface is viewed from a direction of an azimuth 45.degree., the two transmission axis are in a positional relationship at which the angle between them is shifted from 90.degree., the transmitted light generates the birefringence, and light is leaked. As a result, a sufficiently black state is not obtained.
FIG. 1 shows calculated results of the contrast curves of prior LCD devices operated in the IPS mode. The portion, shaded by slanted lines, indicates the area having a contrast more then 50. It is apparent from the figure that a decrease of the contrast occurs at angle 45.degree. from the polarizing axis of one or the other of the polarizing plates, or at the azimuths 45.degree., 135.degree., 225.degree. and 315.degree.. As a result of that the decrease of contrast, an inversion of a luminance (brightness) is generated in the black and gray scale areas.
Therefore, it is an object of the present invention to provide a means for improving the contrast in the 45.degree. directions without decreasing the characteristics in the front direction, the upper/lower direction and the right/left direction in the LCD device operated in the IPS mode which provides the high contrast in the front, upper/lower and right/left directions.